Polyvinyl alcohol (PVOH) is a synthetic resin generally prepared by the alcoholysis, usually termed hydrolysis or saponification, of polyvinyl acetate. Fully hydrolyzed PVOH, where virtually all the acetate groups have been converted to alcohol groups, is a strongly hydrogen-bonded, highly crystalline polymer which dissolves only in hot water—greater than about 140° F. (60° C.). If a sufficient number of acetate groups are allowed to remain after the hydrolysis of polyvinyl acetate, the PVOH polymer then being known as partially hydrolyzed, it is more weakly hydrogen-bonded and less crystalline and is soluble in cold water—less than about 50° F. (10° C.). Both fully and partially hydrolyzed PVOH types are commonly referred to as PVOH homopolymers although the partially hydrolyzed type is technically a vinyl alcohol-vinyl acetate copolymer.
The term PVOH copolymer is generally used to describe polymers that are derived by the hydrolysis of a copolymer of a vinyl ester, typically vinyl acetate, and another monomer. PVOH copolymers can be tailored to desired film characteristics by varying the kind and quantity of copolymerized monomers. Examples of copolymerizations are those of vinyl acetate with a carboxylic acid or with an ester of a carboxylic acid. Again, if the hydrolysis of acetate groups in these copolymers is only partial, then the resulting polymer could be described as a PVOH terpolymer—having vinyl acetate, vinyl alcohol, and carboxylic acid groups—although it is commonly referred to as a copolymer.
It is known in the art that many PVOH copolymers, because of their structure, can be much more rapidly soluble in cold water than the partially hydrolyzed type of PVOH homopolymers. Such copolymers have therefore found considerable utility in the fabrication of packaging films for the unit dose presentation of various liquid and powdered products including agrochemicals, household and industrial cleaning chemicals, laundry detergents, water treatment chemicals, and the like.
The manufacture of packaging films for liquid products poses a unique concern, as they must be free of bubbles and pin holes, and capable of resisting problems such as physical incompatibility exemplified by “weeping”—a condition whereby the package contents seep from the film over a prolonged storage period—caused by certain components typically used in liquid detergent products. The increased level of quality required to manufacture a film and package such liquids adds significantly to the packaged product cost.
Conversely, powdered products are far more forgiving when it comes to packaging. Pin holes and micro bubbles in the film do not present problems for packaging powdered products. Physical compatibility is less problematic, due in part to the non-existence of migrating materials in powdered detergents. Weeping does not commonly occur with the packaging of powdered products. For these reasons the type and quality of PVOH film used to package powered products can be significantly varied from that for the liquid counterparts.
However, those in the powdered product industry have been directed toward the use of either the higher quality, more expensive PVOH copolymer films designed for liquid product packaging or inferior films such as less diverse and more slowly soluble homopolymer-based films including films produced by blown extrusion.
The present invention has solved this problem of film quality versus manufacturing cost as it relates to packaging of powdered product, as well as other problems faced by those in the industry. By making a less expensive PVOH copolymer film having good water-soluble qualities, those in the industry have an alternative source for packaging powdered products.